Step 1 Preheat oven to 150ᵒC fan forced. Line an oven tray with baking paper, set aside. Place egg whites and sugar in a clean, dry bowl and beat with an electric mixer for 10 minutes until mixture is smooth, stiff and glossy.
Step 2 Add sifted cocoa, cornflour and vinegar beat until all combined. Remove bowl and add 6 chopped Tim Tam use a metal spoon to fold in until just combined. Spoon mixture onto prepared tray.
Step 3 Bake for 20 minutes, then reduce oven temperature to 120ᵒC. bake for a further 1 hour and 40 minutes or until shell is pale and crisp. Turn off oven and leave pavlova to cool with the door ajar.
Step 4 Place chocolate in a small microwave safe bowl microwave on 600w power for 1 minute or until melted.
Step 5 Cover pavlova with melted chocolate. Set aside to set. When ready to serve place the pavlova on a plate, top with double, garnish with remaining chopped Tim Tam.